The pathfinder rules skim a little over the idea of adding a monster to a party** and only say that a group of monster PC's should all have the same CR; this is somewhat harder to achieve in practice unless everyone plays the same race.

What is a good mechanic to ensure party balance when the players are playing a mixed bunch of CR creatures?
For example a unicorn (CR 3) a Djinni (CR 5) and a Triton (CR 2) and possibly even a human (CR 1/2 ?)

** and I think include a typo, surely that 8 should be a 5 in the fifth paragraph?

Extra thought for the already excellent answers; for a CR 3 monster with a 4th level party the Monster (3) Class (1) counts it's monster levels as class levels for xp costs; so to get to Monster (3) Class (2) (ignoring bonus levels) it needs XP as if for Class (5)?

At this point the minotaur receives no further bonus levels, because he's received a number equal to half of his CR (i.e. two). Based on this example, it looks like monsters don't receive their bonus levels for levels less than their CR (so the monster doesn't get one between level two and three).

For determining when a monster gains bonus levels, look at the number of levels gained by the party above the monster's CR. In the example the monster is CR 4 so at party level 7 and 10 and extra level is gained, which comes half way through the level. This means that between the 2nd and 3rd party level, and again between the 5th and 6th. For monsters of higher CR this continues every three levels.

Basically when the party level is equal to CR + 3*n -0.5, and n < CR/2, then the monster gains a bonus class level.

Starting Level

[...] Make sure the group is of a level that is at least as high as the monster's CR. Treat the monster's CR as class levels when determining the monster PC's overall levels.

In your case this means you have a level 5 party (the highest CR).

The Djinn is pretty simple: He's a level 5 Djinn. He's entitled to two bonus levels (half of five, rounded down), which he'll receive at level 7.5 and 10.5.

The Unicorn is entitled to one bonus level, which he'll receive at level 5.5. At 5th level, he'll be a Unicorn 3/ <class level> 2.

The Triton is entitled to one bonus level, which he'll receive at level 4.5. At 5th level, he'll be a Triton 2 / <class level> 4.

The human is the easiest: Humans don't have racial hit dice, so they just use the normal rules.

Experience to Level

This is where things get a little tricky. The rules are phrased in terms of overall party level, which simplifies things a great deal. When you're talking about the level of a specific character, it's more complicated.

In terms of the party, a CR3/Class1 monster fights with a fourth level party. It is effectively a fourth level character in terms of XP. When the party reaches level five, it levels up to CR3/Class2. So your edit is correct.

Basically, in terms of leveling, calculate the monster's level like this:

So in the Minotaur example above, the Minotaur is a Minotaur 4 / Barbarian 8. For experience totals, he's a tenth level character. He levels up when he acquires enough experience points to reach eleventh level. At that point he becomes a Minotaur 4 / Barbarian 9.

I think the typo is "when the group is at a point between 6th and 7th level, the minotaur gains a level, and then again at 7th, making him a minotaur barbarian 4.. I think the Barbarian 4 is the typo. I think that should be 6 or 7. It would be better to just say that the monster PC levels every party 2.5 levels until level 10.

I think RAW it is telling you that the starting level of any monster PC is its CR. Add half of that as a core Multiclass.

In order for a character to be a Monster PC, it would have to have a class (barbarian, fighter, etc.) Since Monsters have inherent power because they are monsters, you have to be careful to not add further monster levels too fast.

So as the example in the RAW says, a party of 10th level PCs that has a monster in it, the monster (Minitaur in the example) should be level 8 (Monster4/Class4). In the example, I think the Minitaur is a Barbarian 8 (with four inherent Monster levels increases from level 4 start (one level every 2.5 party levels although it is written wrong in the rule). So the monster would level up from 4 at party level 2.5, 5, 7.5, and 10. But from then on, it levels up as the party would. Not sure why on that.

In your case, the starting level of the party would have to be 5 as that is the starting level of your party. So you would have to level up the other party members.

Not sure that is right or not, as I have not done this yet, but that is how I read it even though again it is not written too well.